Human immunity to Schistosoma mansoni: observations on mechanisms, and implications for control.

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IMMUNOLOGICAL INVESTIGATIONS, 21(5), 391-407 (1992)

HUMAN IMMUNITY TO SCHISTOSOMA MANSONI: OBSERVATIONS ON MECHANISMS, AND IMPLICATIONS FOR CONTROL Anthony E. Butterworth, David W. Dunne, Anthony J. C. Fulford, and Kareen J. I. Thorne Division of Microbiology and Parasitology Department of Pathology, Tennis Court Road, Cambridge CB2 1 QP, UK. Kimani Gachuhi Kenya Medical Research Institute, P.O. Box 54840, Nairobi, Kenya. John H. Ouma Division of Vector Borne Diseases Ministry of Health P.O. Box 20750, Nairobi, Kenya. Robert F. Sturrock Department of Helminthology London School of Hygiene and Tropical Medicine KeppeI Street London WClE 7HT,UK.

ABSTRACT This review summarizes the personal experiences of the authors and their colleagues during ten years of field and laboratory studies on human immunity to Schistosoma mansoni infections. There is evidence for the very slow development with age of an acquired resistance to reinfection (demonstrable after chemotherapy of the primary infection) distin391 Copyright 0 1992 by Marcel Dekker, Inc.

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guishable from a lack of reinfection due to reduced exposure. The implications of this immunity for the design of chemotherapy programs targeted at infected schoolchildren are discussed. Observational studies on the mechanisms of immunity have demonstrated a marked helminthocidal capacity of eosinophils. Subsequent correlative studies have indicated a role of IgM and IgG2 “blocking” antibodies in maintaining the continued susceptibility of young children, and of IgE antibodies in mediating protection in older individuals. Some problems in studying human immunity, and the implications for vaccine development, are also discussed.

INTRODUCTION Safe and effective drugs for the treatment of uncomplicated schistosome infections have been available for almost two decades. The remaining challenges in schistosomiasis are found less in the clinical management of individual patients, but rather in the control of transmission and of morbidity at the community level. Although avariety of measures may be adopted in an integrated approach, including vector control, sanitation, health education, and the provision of water supplies, the mainstay of control at present is repeated mass or targeted chemotherapy. This can be effective, but is expensive in both drug costs and the use of skilled manpower in diagnosis and delivery of treatment. It also requires the establishment of vertically-structured, disease-specific control programs with surveillance and retreatment continuing indefinitely. Such programs are difficult to fund and maintain and, although chemotherapy is a useful interim control measure, alternative approaches would be desirable in the long term, among which vaccination must be considered as a possible candidate. Within this framework, several questions are of practical concern to the immunologist. First, does acquired immunity limit the extent of reinfection after treatment and, if so, what are the consequences of such an immunity for the design of chemotherapy control programs? Secondly, if an acquired immunity can be demonstrated to occur in humans, what are its mechanisms and what are the implications for vaccine development? And thirdly, since the aim of control is the reduction of disease rather than simply of infection or transmission, what immunological processes may contribute to the wide variation that is observed between infected individuals in the clinical severity of disease? Attempts to answer these questions may provide information that is of value not only in the narrow context of schistosomiasis control, but also in more fundamental understanding of the processes involved in immunity to helminth infections. This article is not intended as a comprehensive review of the field, but rather as a summary of the particular experiences of the authors and their colleagues in studying immunity to S. mansoni infections within the framework of a large-scale pilot chemotherapy control program in Kenya.

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EVIDENCE FOR IMMUNITY TO SCHISTOSOMA MANSONI IN HUMANS The characteristic convex distribution by age of the prevalence and intensity of schistosome infection in communities living in endemic areas suggests that, in older individuals, a slow attrition of adult worms from early infections is accompanied by a slowly-acquired immunity to new infections (1). However, the same pattern of infection would be seen if a slow death of adult worms were accompanied by reduced levels of exposure to infection among older individuals. In addition, the rate of loss of adult worms is not accurately known and may be variable: and the possibility exists that egg output by such worms may be suppressed. Simple examination of age-specific prevalence or intensity curves can therefore provide no definite information about the existence or extent of acquired immunity, although various refinements of the basic observation of the decline in infection with age have strongly indicated that such an immunity may occur (reviewed in Ref. 2). In an attempt to obtain unambiguous evidence for the existence of immunity, we have carried out a series of studies over a ten-year period in which we have followed intensities of reinfection after chemotherapeutic cure in groups of individuals whose levels of contact with contaminated water have also been observed (3, and Fulford et al., in preparation). Similar studies have also been undertaken by Wilkins and colleagues on S. haematobiurn in The Gambia (4), and by Dessein and colleagues on S.mansoni in Brazil (5, and this volume), with essentially similar conclusions. Following treatment, the extent to which reinfection subsequently occurs is strongly dependent upon age. In conditions of continued transmission, young children rapidly become heavily reinfected: In some cases, intensity of infection (as reflected by egg excretion in the feces) may reach 50%of pretreatment levels within one year (6). In contrast, older children and adults become reinfected, if at all, only at very low intensities. Furthermore, the relative lack of reinfection among older children and adults cannot be attributed simply to a lack of exposure. In contrast to reinfection intensities, which reach a peak in children aged 8 to 12 years, the extent of water contact does not peak until the ages of 16 to 24 years (3). Even when the water contact data are weighted to take into account the possibility that young children have a different and less infective type of water contact, the slight decline with age in weighted water contact is still completely insufficient to account for the very marked lack of reinfection among older individuals. For example, in one community, we found that young adults aged 20 to 24 years had over half the level of “weighted exposure” that was observed in susceptible children aged 8 to 12 years, but only 1%of the mean level of reinfection - implying, although admittedly in oversimplified terms, a 98% level of immunity (3). Additional comparisons of reinfection after treatment among previously infected adults with new infections occurring among previously uninfected adults from the same community have indicated that the lack of reinfection in treated adults does depend on previous experience of infection, and therefore reflects an acquired


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immunity. However, the additional involvement of simple, age-dependent, physiological changes cannot at this stage be excluded. Determination of the relative contributions of acquired immunity, age-dependent physiological changes, and agedependent changes in exposure will depend on studies on different types of communities - in particular, on adult immigrants to endemic areas, as originally described by Kloetzel and da Silva (7). Such studies are planned. For the moment, however, we conclude that an acquired immunity does develop in older individuals and plays a major role in limiting new infections or reinfection after treatment, although it is not possible to make a quantitative estimate of the efficacy of such immunity.

IMPLICATIONS OF ACQUIRED IMMUNITY FOR THE DESIGN OF CHEMOTHERAPY CONTROL PROGRAMS The most effective way to reduce transmission of S.mansoni, and hence its associated morbidity, is to treat, on a repeated basis, all infected individuals in a community; and, following the seminal observations of Jordan and colleagues in St. Lucia (reviewed in Ref. 8), many research studies and operational control programs have relied on this approach. Unfortunately, as well as being effective, it is also extremely expensive in both drug costs and skilled labor required for diagnosis and delivery of treatment at the community level. Since costs and availability of skilled technical staff are major factors in determining control strategies in most developing countries, alternative approaches to chemotherapy of all infected individuals that are both less expensive and less labor intensive would be desirable. One possibility that has been proposed and tested in the past is to treat only those individuals with heavy infections (9), on the grounds that, since the development of disease is related to the number of eggs deposited in the tissues, and hence to the number of (nondividing) adult worms harbored by the host, those with heavy infections are most likely to deveIop severe disease. This approach leads to some reduction in drug costs, but the major expenses that are involved in carrying out examination and retreatment at the level of the whole community are unchanged and high. An alternative approach, suggested partly by the existence of an acquired immunity among adults, is to target treatment towards young children. This group is most susceptible to infection and rapidly acquire high worm burdens. They include the most heavily infected individuals in a community, and are the most severely affected in terms of both symptoms and clinical signs. Although a few patients may progress towards severe and potentially fatal hepatosplenomegaly with portal hypertension and oesophageal varices in late childhood or adult life, the early stages of this condition develop while the child is young, and spontaneous regression of organomegaly is common. Young children also contribute most to contamination of the environment, by indiscriminate defecation in and around the waterbodies, and



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they are easily and readily accessible through the framework of the primary schools. In addition, the lack of immunity among this group implies that they will become most rapidly reinfected and should therefore form the target for surveillance and retreatment campaigns. A possible approach to control, therefore, is to carry out campaigns of stool examinations and treatment in the primary schools only, together with offers of treatment to those sick adults who present spontaneously. This approach naturally depends on the primary school attendance rate in any given society. In Kenya, where school attendance is high, we have.found that treatment of schoolchildren only is a highly effective method of reducing both morbidity and transmission in the community as a whole. The approach, which requires that surveillance and retreatment in the schools be carried out at intervals of two to three years, is manageable and feasible enough that it can be recommended for adoption in a national control program (2,54).

MECHANISMS OF IMMUNITY In the short term, chemotherapy targeted at schoolchildren is a feasible basis for control at the national level. In the longer term, however, it is not ideal. Transmission, although reduced, can never be abolished; and, since young children do become reinfected, surveillance and retreatment must be carried out indefinitely. In addition, this type of approach to control must be carried out within the framework of a disease-specific, "vertically-structured" program organized centrally from a Ministry of Health, and such programs are notoriously difficult to maintain and fund over long periods of time. A preferable alternative measure, even if used only as an adjunct to control, would be by immunization, a procedure that could be incorporated into existing (and usually highly effective) expanded programs of immunization (EPI) at the primary health care level. The demonstration by ourselves and others that humans do show an immunity to reinfection after treatment supports the idea derived from the increasing number of successful vaccination studies in experimental animal models that it should be possible to develop a vaccine for use in humans. However, a prerequisite for successful vaccine development is not only that an appropriate vaccine antigen is available, but also that it can be used in a manner that will elicit aprotective immune response. The extensive studies in animal models, reviewed elsewhere in this volume, have provided useful pointers toward the types of immune responses that might be protective in humans. However, each animal model differs quite markedly from the next in both the extent and the nature of the protective immune responses that can be demonstrated. None can be taken a priori to reflect the situation in humans and, for the development of a vaccine, an understanding of those responses that are protective in humans is essential. The search for such responses has also led


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to the discovery of phenomena that are of more general immunological or biological interest. In particular, we have been interested in a possible role in human immunity of reactions involving eosinophils and IgE. It has been recognized for many years that there is a strong association between helminth infections and, on the one hand, increased total and parasite-specific IgE levels and, on the other hand, increased numbers of circulating eosinophils. A similar association is seen in a wide range of allergic disorders, including asthma, in which there is evidence to suggest that eosinophils may contribute to the pathogenesis of host tissue damage. In contrast, there is extensive evidence from the studies of Capron and colleagues (10, and this volume) that IgE-mediated reactions, including those involving eosinophils, can contribute to immunity against helminth infections. The description in the mouse of a subset (Th2) of Tcells that regulates both eosinophils and IgEproduction, through IL-4 and IL-5, indicates a reason for the association between eosinophilia and IgE. Although it is currently being argued that, in the mouse, ThZrelated events are associated with pathogenesis of disease and Thl-related events with immunity (1l), this would not appear to be the case in the rat (12). In humans, the evidence accumulated so far, summarized below, is beginning to suggest that, although a clearcut identification of Th2-dependent events cannot yet be made, at least the “eosinophil/IgE axis” may be involved in protective immunity.

Eosinophil-Mediated ADCC Reactions Early work in experimental animal models suggested that the young schistosomulum - the stage of the parasite that is found immediately after the infective cercaria penetrates the skin of the mammalian host and loses its waterresistant outer coat or glycocalyx- is a major target of immune attack (13). Adult worms, in contrast, are refractory to immune attack, and continue to survive even in experimental hosts that have developed an immunity to reinfection with a secondary challenge of fresh cercariae (14). Schistosomula can be prepared simply by allowing the infective cercariae, recovered from infected snails, to penetrate an isolated preparation of rat or mouse skin in an appropriate chamber, and can then be maintained in culture for prolonged periods (15). It is then possible to test such organisms, cultured in vitro, for their susceptibility to damage by a range of human immune effector mechanisms that might be candidates for being “protective” human immune responses, in a manner analogous to the use of virus-infected target cells in vitro for the study of candidate anti-viral immune responses. Initially, experiments involving human effector responses in vitro were carried out at the descriptive level, without attempting to relate such responses to the expression of immunity in vivo. During the 1970s, a range of human immune responses of varying levels of effectiveness were reported to be capable of killing schistosomula in vitro (reviewed in Ref. 16). Antibody and complement alone, without added effector cells, can mediate some damage, but the effect is slight (15). Cytotoxic lymphocytes, including both cytotoxic T lymphocytes and NK cells, are



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generally considered to be inactive (17), although there are some reports of lymphocyte-mediatedkilling (18). Instead, most attention has been focussed on the antibody-dependent,cell-mediated killing that can be observed with macrophages, platelets, neutrophils, and eosinophils. Macrophages and platelets can both mediate damage to schistosomula in the presence only of IgE antibodies, as described by Capron and colleagues (19, and reviewed in this volume). In our hands, neutrophils are unable to mediate damage (20), although such damage has been reported by others (21). In contrast, eosinophils are highly active in killing schistosomula in the presence of IgG (22-24) or, in some circumstances, IgE (25) antibodies. The preferential effect of eosinophils, in comparison with neutrophils, is associated with the particular capacity of these cells to degranulate upon contact with large, nonphagocytosable, antibody-coated surfaces and to release onto such surfaces their granule contents, especially the toxic cationic proteins MBP and ECP (26,27). In addition, early experiments showed that eosinophils recovered from the blood of patients with raised peripheral blood eosinophil counts, associated with helminth infections or allergic disorders, showed an enhanced capacity to kill schistosomula (28), and may be referred to as activated. Such activation may be induced both by a variety of known cytokines with other functional properties, including tumour necrosis factor, TNF (29), platelet activatingfactor, PAF (30), and the colony-stimulating factors GM-CSF (31), IL-3(32), and IL-5(33), and also by other monocyte or lymphocyte-derived mediators, including eosinophil activating factor, EAF (34,35), and eosinophil cytotoxicity enhancing factor, E-CEF (36). In recent studies, we have investigated some aspects of the interactions between these mediators and their effects on eosinophil function. Activation is reflected by an alteration in a broad range of functional properties. These include morphological changes, with the formation of lamellopodia and an increased capacity to spread on glass, increased expression of cell surface receptors for C3 and IgG, increased oxidative metabolism, and an enhanced capacity to degranulate in response to an appropriate trigger (37). In experiments in which the enhanced expression of CDllb (the a-chain of the CR3 receptor) has been used as a marker for activation, we have found that IL-3, GM-CSF, and IL-5 act independently, whereas TNF, EAF, and PAF are synergistic in their effects (Thome et al., unpublished observations). In addition, the effect of E M , and to a lesser extent of TNF, can be inhibited by PAF antagonists, suggesting that TNF and EAF may act by inducing the formation of PAF, the PAF subsequentlyhaving an autocrine effect. These various studies, camed out over a period of some 15 years, have shown that human eosinophils have a specialized capacity to kill antibody-coated schistosome larvae (as well as various other helminth targets), a property that is enhanced in conditions of activation. However, the fact that eosinophil-mediated reactions are particularly potent effector mechanisms against helminths does not necessarily imply that they are actually involved in the expressionof immunity as it occursunder conditions of natural infection in vivo. Support for (although not proof of)such arole would come from studies that demonstrate a positive correlation between the

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expression of immunity and the presence of activated eosinophils and/or antibodies that mediate eosinophil-dependent killing of schistosomula in vitro. Early studies by Sturrock et al. (38) demonstrated a correlation between high eosinophil counts and lack of reinfection after treatment of S. rnansoni infections, and a similar result was clearly demonstrated for S. haemarobium by Hagan and colleagues (39). However, more recent and detailed studies in Kenya on small cohorts of children have failed to demonstrate a simple relationship. Instead, a more complex process appears to be involved, in that the presence in younger children of inappropriate or “blocking” antibodies may prevent the expression of eosinophil-mediated protective responses (40). This hypothesis is described in more detail below.

Role of Blocking Antibodies Although children in endemic areas start to acquire schistosome infections from the age of 3 or 4 years, they do not become demonstrably immune (as reflected by a lack of reinfection after treatment in spite of high levels of exposure) until they are at least 12 years old. During the intervening period, they are progressively accumulating infections; and, if they are treated, they rapidly become heavily reinfected. However, this lack of immunity among younger children cannot be attributed simply to low overall levels of immune responses (for example, as a result of low antigenic loads during the early years of cumulative infections). All children, including the younger ones who remain susceptible to reinfection after treatment, show a wide range of immune responses to antigens from various parasite stages. Studies on antibody responses in such children yielded initially no correlates of immunity- that is, of situations in which high levels of a particular antibody in pretreatment or immediate post-treatment blood samples were associated with low subsequent intensities of reinfection. Instead, some antibody responses were found to correlate strongly with susceptibility to reinfection (40-42). These included particularly antibodies of the IgM and IgG2 isotypes that recognized carbohydrate epitopes expressed both in eggs and on the surface of the developing schistosomula. These findings led to the hypothesis that, during early infections in young children, the major immunogenic stimulus is the large quantity of eggs that are deposited in the tissues (rather than the schistosomula and adult worms, which are present in relatively much smaller mass). Polysaccharide antigens released from eggs elicit predominantly a T-independent antibody reponse, comprising especially IgM and IgG2 antibodies. These antibodies cross-react with carbohydrate epitopes on polysaccharides or glycoproteins at the schistosomulum surface, and not only fail to mediate ADCC reactions by eosinophils or other cells, but may indeed block the binding of antibodies of “effector” isotypes, separately elicited against the same or closely adjacent epitopes. These blocking antibodies then decline with age, permitting the binding and action of effector antibodies and hence the expression of immunity. The existence of such blocking antibodies had previously been demonstrated in the rat model of schistosomiasis, in which Grzych et al. (43) had found that IgG2c monoclonal antibodies would block the effect of an IgG2a monoclonal antibody, with specificity for the same antigen, both in vitro and in vivo.



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Both IgM and IgG2 antibodies are high in young children and subsequently decline with age and, in studies involving the CBA/N mouse, deficient in type 11Tindependent responses, it has been found that egg antigens elicit T-independent responses of the IgM and IgG3 classes (44). In addition, Khalife et al. (45) demonstrated directly that purified IgM would block the eosinophil-dependent killing of schistosomula mediated by IgG antibodies from the same sera. Subsequently,following purification of individual IgG subclasses, he found that IgGl and IgG3would mediate eosinophilkilling of schistosomula. IgG4 consistentlyblocked killing, while IgG2 would either mediate or block killing, depending on the state of activation of the eosinophils.

Putative Role of IgE and Other Isotypes The observations summarized above were compatible with the hypothesis that the expression of immunity in young children was limited by the presence of inappropriate or “blocking” antibodies that prevented the binding and action of appropriateprotective antibodies. However, in these early studies, there was no clear indication as to the nature of the protective antibodies, in that no positive correlation was found between any antibody response and the expression of immunity. Our further studies on this question have been stimulated by recent observations by Hagan and colleagues on S. haematobium infections (46). They have show that IgE antibodies increase progressively with age (in contrast to IgGl and other isotypes), and, after allowing for age and exposure in a logistic regression analysis, they have shown that resistance to reinfection is significantly related to the levels of IgE antibodies to adult worm and (to a lesser extent) egg antigens. In contrast, IgG4 antibodies in the same analysis are significantly related to susceptibility to reinfection. The observed effects are marked, those with the highest IgE levelsbeing ten times less likely to become reinfected after treatment (after allowing for age and exposure) than those with the lowest levels. These findings have provided the first evidence to support the idea that IgE plays a protective role in mediating immunity to helminth infections in humans. We have subsequently undertaken similar studies, in the context of S. mansoni infection, in a new study group of 140individuals of wide age range, treated in 1985 and followed for reinfection over a three-year period (55). We have found that IgE antibodies against adult worm antigens increase with age and are significantly correlated with a lack of subsequent reinfection. In contrast, IgE antibodies against antigens prepared from eggs or young schistosomula, show no increasing trend with age, and no correlation with reinfection. The relationship between IgE anti-adult worm responses and resistance to reinfection holds true for responses against periodate-insensitive epitopes, suggesting that (in contrast to the IgM and IgG2 associations previously described) the relevant epitopes are not carbohydrate in nature. The demonstration, both by Hagan and colleagues and subsequently by Dessein and colleagues (56) and by ourselves (55) of an association between IgE

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responses and resistance to reinfection, is particularly marked and fits neatly into the idea of a protective role for IgE. In addition, however, we also have evidence for an association between lack of reinfection and IgA responses against adult worm antigens. These IgA responses increase particularly during adult life (from the age of 25 years onward) rather than during late childhood as in the case of IgE responses. They might contribute to the very marked immunity seen in adults.

UNRESOLVED QUESTIONS CONCERNING IMMUNITY AND MORBIDITY Our findings on S.mansoni summarized in the previous section are encouraging in two respects. First, they provide a plausible explanation both of the mechanism of immunity and of the reason why immunity takes so many years to develop. Secondly, they add support to the emerging evidence that IgE may indeed be involved in mediating immunity in man. However, many aspects are still unclear or untidy, and are currently under study. This section summarizes work in progress on some of these unresolved issues. Our serological studies so far have involved the use of unpurified antigenic extracts of different life cycle stages (schistosomula, adult worms, and eggs). This has been deliberate.With an organism as complex as the schistosome, it is extremely unlikely that immunity will be determined by responses to a single antigen. Instead, the net level of immunity is more likely to depend on a combination of different responses to different antigens. It is of course valid to opt to study responses to individual purified or recombinant antigens, and this can provide useful information. For example, as described elsewhere in this volume, significant correlations are observed between the presence or absence of immunity and responses to the 28,000 Mr glutathione-S-transferase.These responses include IgA antibodies (47), and also the balance between IgE and IgG4 antibodies, with IgG4 being associated with susceptibility to reinfection. However, it is equally valid to opt to study heterogeneous responses to heterogeneous antigen preparations, if one is lucky enough to demonstrate correlations between immunity and the net level of such heterogeneous responses. This has been the case with our results, in which we have found an association between IgE responses and the expression of immunity. The next stage is to see which, if any, antigens are selectively recognized by IgE antibodies from the older immune individuals. Such antigens may include a 22,000 Mr molecule (55). A second problem is to determine the relationship between the various correlations that we have observed. Our original observations on eosinophilmediated ADCC reactions and on the possible role of blocking antibodies depended on the hypothesis that the young schistosomulum is the main target for immune attack. However, our later observations on IgE antibodies have shown that it is the IgE response against antigens expressed in adult wonns (and possibly older schistosomula, which we have not yet tested) that correlates positively with the



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expression of immunity, rather than IgE responses against antigens in young schistosomula. It is possible, and indeed likely, that a combination of different events will contribute to the expression of immunity. Young schistosomula may be the target for eosinophil-mediated ADCC attack, an event that is prevented by blocking antibodies (which may indeed promote the migration of schistosomula from the skin). Subsequently, older schistosomula, at some stage during their migration from the skin through the lungs to the hepatic portal system, may be the target of (unidentified) effector mechanisms that depend on the presence of IgE antibodies. The findings presented in this article indicate that “ThZ-1ike”effects(involving IgE and/or eosinophils, and possibly IgA) may be important in mediating human immunity to schistosome infection. This does not of course exclude the possibility that, as in the mouse model of immunity, “Thl-like” effects may also be important. However, preliminary observations have failed to demonstrate an association between gamma-interferon production in response to an adult worm antigen preparation and either age or resistance to reinfection after treatment (Butterworth et a]., unpublished observations). This is now being studied in considerably more detail, by examining the pattern of cytokine production (IL-2, gamma interferon, IL4, IL-5, and TNF) by lymphocytes of a group of heavily-exposed individuals aged 10 to 40 years, in response to schistosomulum, egg and adult worm antigens as well as various recombinant antigens. Full results are not yet available, but a striking feature is the high proportion of individuals whose lymphocytes produce IL-5 in response to various antigens (Roberts et al., unpublished observations). Various other notes of caution must be kept in mind when interpreting studies on human immunity. First, although the best design of such studies currently available is to follow intensities of reinfection after chemotherapy, it has the intrinsic disadvantage that chemotherapy itself may induce immunological changes, and that the events observed may be different from those in infected and untreated people. This does not invalidate the design - and indeed, since chemotherapy is widely used in control, it is of considerable practical importance to know about immune status after chemotherapy -but care should be taken to avoid over-generalization. Secondly, the finding that immunity is so strongly age-dependent makes it extremely difficult to control for age when investigating the importance of any given immune response. One possible approach to this problem, if the age effect simply reflects duration of exposure, is to investigate adult immigrants to endemic areas, as originally done by Kloetzel and da Silva (7). However, if age itselfis important (for example, changes in immune status dependent on puberty), then it may prove impossible to overcome the problem. Thirdly, all observations on putative human immunity (implying resistance to the development of adult worms of a challenge infection) depend on the hypothesis that, in humans, egg output reflects worm burden. For S. mansoni infections at least, there is no evidence to suggest that this is not the case, although it might be speculated that (by analogy with rodent models) the effect of IgA in older adults is to reduce egg output in some manner (reduction in either fecundity or excretion in the feces). An approach to this problem could be to relate, for different age groups, the excretion of eggs in the feces with another


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parameter that may more accurately reflect worm burden, such as the levels of the circulating anodic antigen (CAA) described by Deelder and colleagues (48). Although still indirect, such a correlation, if found, would support the hypothesis that egg output reflects worm burden. Such a study is planned. Finally, from the practical point of view, it should be remembered that the aim of any control measure, such asvaccination, is to reduce morbidity. As in thevarious animal models, immunity in humans appears to be frequently incomplete“immune” adults often do become reinfected, but at lower intensities than “susceptible” children. Many studies have shown that the severity of clinical disease is dependent on intensity of infection rather than simply its presence or absence (4951), implying that even an incomplete immunity may be of considerable value. However, it should also be kept in mind that intensity of infection may not be the only determinant of disease. Although outside the main framework of this review, we have recently identified an area of high schistosome-associated morbidity that is not solely due to high intensity of infection (2,52), and we are investigating some of the factors with which this morbidity may be associated. These may include host or parasite genetic factors, the nature of exposure, interactions with other infections or with nutritional status, and the possible role of maternal infection in the subsequent development of morbidity in the offspring (53).

CONCLUSIONS: IMPLICATIONS FOR VACCINE DEVELOPMENT The findings summarized in this chapter, like those presented elsewhere in this volume, bode well for the eventual development of a vaccine for use against schistosome infection in humans. Immunity can be demonstrated in naturally infected individuals after subsequent chemotherapy, and the indications are that it is the nature of the response that is important, rather than the exact identity of a particular “protective” antigen (although peptide antigens appear to be involved, encouraging further experiments with peptides prepared by recombinant DNA techniques). The crucial issue appears to be the achievement of the right balance of protective, as opposed to inappropriate (or even antiprotective), responses. Considerable work will be needed on how to elicit the “correct” response in humans through the use of acceptable adjuvants. In addition, much more work is needed on other aspects of experimental and human immunity, including the impact of chemotherapy, the effect on an incomplete vaccine-induced immunity of subsequent low level infections, and the relationship between intensity of infection and other factors that may determine morbidity. Therefore, even with the emerging availability of a variety of candidate recombinant vaccine antigens, their widespread use in humans must be seen as resting some time in the future. In the meantime, however, the study of immunity during human schistosome infections has led to information that may be considered to be of general biological interest, including particularly the effector function of eosinophils and the apparent key role in immunity of IgE antibodies.


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ACKNOWLEDGEMENTS We thank all our colleagues in Kenya and in Europe, whose published and unpublished observations are referred to in this review. We would also particularly thank Dr. P. Hagan and colleagues (46) for permission to summarize unpublished work. Original work by the authors and their colleagues, summarized in this review, have been supported by grants from the Edna McConnell Clark Foundation, the Medical Research Council, the Commission of the European Communities, the Rockefeller FoundationflDR Joint Funding Venture, and the Asthma Research Council.

REFERENCES 1. Clarke, V.de V. 1966. Evidence of the development in man of acquired resistance to infection of Schistosoma spp. Cent. Afr. Med. J. 12 (Suppl. 1):l-30. 2. Butterworth, A. E., Corbett, E. L., Dunne, D. W., Fulford, A. J. C., Gachuhi, R. K., Klumpp, R., Mbugua, G., Ourna, 3. H., arap Singok, T. K., and Sturrock, R. F. 1989. Immunity and morbidity in human schistosomiasis. In “New Strategies in Parasitology.” K. P.W. J. McAdam (Ed.). Churchill Livingstone, Edinburgh, p. 193. 3. Butterworth, A. E.,Fulford, A. J. C.,Dunne, D. W., Ouma, J. H., and Sturrock, R. F. 1988. Longitudinal studies in human schistosorniasis. Phil. Trans. Roy. SOC.Lond. B 321~495-511. 4. Wilkins, H. A., Blumenthal, U. J., Hagan, P. A., Hayes, R. J., and Tulloch, S. 1987. Resistance to reinfection after treatment of urinary schistosomiasis. Trans. Roy. SOC. Trop. Med. Hyg. 81:29-35. 5 . Dessein, A. J., Begley, M., Demeure C., Caillol, D., Fueri, J., Galvao dos Reis, M., Andrade, 2.A., Prata, A., and Bina, J. C. 1988. Human resistance to Schistosoma mansoni is associated with IgG reactivity to a 37-kDa larval surface antigen. J. Immunol. 140:2727-2736. 6. Butterworth, A. E. 1988. Control of schistosomiasis in man. In The Biology of Parasitism. P. T. Englund and A. Sher (Eds.). Alan R. Liss, New York, p. 43. 7. Kloetzel, K, and da Silva, J. R. 1967. Schistosorniasis mansoni acquired in adulthood: behaviour of egg counts and the intradermal test. Amer. J. Trop. Med. Hyg. 16:167169. 8. Jordan, P. 1985. Schistosorniasis-theSt. Lucia project. Cambridge University Press, Cambridge. 9. Mahmoud, A. A. F., arap Siongok, T. K., Ouma, J. H., Houser, H. B., and Warren, K. S. 1983. Effect of targeted mass treatment on intensity of infection and morbidity in schistosomiasis mansoni. Lancet i:849-851. 10. Capron, A., Dessaint, J. P., Joseph, M., Torpier, G., Capron, M., Rousseaux, R., Santoro, F., and Bazin, H. 1977. IgE and cells in schistosomiasis. Amer. J. Trop. Med. Hyg. 26 (Suppl.):3947. 11. James, S. L., and Sher, A. 1990. Cell-mediated immune response to schistosomiasis. Current Topics Microbiol. Immunol. 155:21-31.


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Lung inflammation in immunity to Schistosoma mansoni.

Human IgE responses to Schistosoma mansoni and resistance to reinfection.

Further observations from St Lucia on control of Schistosoma mansoni transmission by provision of domestic water supplies.

Protective immunity to Schistosoma mansoni in mice is dependent on antibody and complement but not on radiosensitive leukocytes.

T cell derived cytokines in lung-phase immunity to Schistosoma mansoni.

Different levels of immunity to Schistosoma mansoni in the mouse: the role of variant cercariae.

Schistosoma mansoni antigen detects Schistosoma mekongi infection.

Evidence for an association between human resistance to Schistosoma mansoni and high anti-larval IgE levels.

The effect of praziquantel on Schistosoma mansoni.

Secondary amyloidosis due to Schistosoma mansoni infection.

Granulomatous oophoritis due to Schistosoma mansoni.

Stereoscan observations of the miracidium and early sporocyst of Schistosoma mansoni.

Human schistosomiasis: Schistosoma mansoni antigen detection in renal glomeruli.

Acquisition of human blood group antigens by Schistosoma mansoni.

Schistosoma mansoni: acquired immunity in mice and hamsters using antigens of Fasciola hepatica.

A benzodiazepine derivative and praziquantel: effects on musculature of Schistosoma mansoni and Schistosoma japonicum.

Pathogenesis of Schistosoma mansoni infection.

Evaluation of an experimental mollusciciding programme to control Schistosoma mansoni transmission in St Lucia.

A socioecological approach to the control of Schistosoma mansoni in St Lucia.

Schistosoma mansoni and Schistosoma japonicum: effects of infection on induction of tryptophan oxygenase in mouse livers.

The prolonged use of niclosamide as a molluscicide for the control of Schistosoma mansoni.

Infarction of testicle and Schistosoma mansoni.

The effect of ultra-violet radiation on cercariae of Schistosoma mansoni and Schistosoma haematobium.

The role of antibody affinity and titre in immunity to Schistosoma mansoni following vaccination with highly irradiated cercariae.